Answer: Use damp cloth with water, or use eraser. If you ironed marked line and found a difficulty to remove marked line, wash it by hand with warm water with soap. Detergent is also helpful to remove the marked line. Lead is colored with Dye which is very special and weak against sun light and color will fade under sun light.
Answer: Use needle to clean inside of tip, the residue of lead is removed by cleaning inside mechanism.
Please do not put too many leads into the lead chamber. 2 or 3 leads would be limit number of leads for the chamber.
Answer: No, do not mix any different color leads in a chamber of pencil or lead case as they may chemically react each other.
Keep a lead chamber with single color leads.